1. Field of the Invention
The present invention relates to a light-emitting device and particularly to a light-emitting device including a semiconductor layer having a portion formed of a group III nitride semiconductor.
2. Description of the Background Art
A light-emitting diode representing a light-emitting device disclosed in Japanese National Patent Publication No. 2002-527890 has a group III nitride semiconductor layer on a silicon carbide substrate. This light-emitting diode is constructed as a vertical type by making use of conductivity of the silicon carbide substrate. In addition, since lattice unmatch between silicon carbide and a group III nitride semiconductor is small, a group III nitride semiconductor layer can be grown on the silicon carbide substrate with small strain, and consequently a high-performance light-emitting diode is obtained.
In order to enhance quantum efficiency of a light-emitting device, quality of the silicon carbide substrate on which a semiconductor layer is to be formed should be enhanced, and in particular dislocation density should be lowered. Here, dislocation density refers to the number of dislocations per substrate area. Examples of dislocation include basal plane dislocation, threading edge dislocation, threading screw dislocation, and mixed dislocation, or a hollow threading defect called micropipe.
In addition, in order to lower ON resistance for enhancing power efficiency of a vertical type light-emitting device, electric resistance originating from the silicon carbide substrate should be lowered. Though this resistance can be lowered by decreasing a thickness of the substrate, the thickness of the substrate cannot excessively be decreased in order to sufficiently ensure a function for the substrate to support the semiconductor layer. Therefore, in order to sufficiently lower this resistance, electrical resistivity of the silicon carbide substrate should be lowered.
As described above, in order to enhance both of quantum efficiency and power efficiency, dislocation density and electrical resistivity of the silicon carbide substrate should both be lowered. It is difficult, however, to achieve both of these in terms of physical property of silicon carbide, and hence it has been difficult to enhance both of quantum efficiency and power efficiency of a light-emitting device.